Removal of trace amount impurities in glycolytic monomer of polyethylene terephthalate by recrystallization

Abstract

Chemical recycling is a promising way to achieve the closed-loop recycling for polyethylene terephthalate (PET). During this process, the post-consumer PET is depolymerized into bis(2-hydroxyethyl) terephthalate (BHET) through glycolysis and BHET can be polymerized again to produce recycled PET (rPET). However, since the applications of rPET is greatly limited to the color, recycling of PET fibers is challenged by the residual of colored impurities. In this study, a modified recrystallization process using ethyl acetate (EA) as the solvent was employed for the removal of colored impurities from the BHET. The process showed a decoloring rate of over 97.5% for the model colored impurities, performing better than the reported physical adsorption processes. Since almost no disperse dye was precipitated during the nucleation and crystal growth process due to the high solubility in EA, only a small amount of colored impurities adsorbed on the surface of crystals, leading to an excellent decolorization performance and post-consumer EA could be easily recycled through evaporation followed by condensation. Besides the model colored BHET, raw BHET obtained from the depolymerization of dark PET fibers was also purified by this modified recrystallization. By applying a composite purification strategy combining EA-based recrystallization and activated carbon adsorption, the whiteness (L*) of BHET greatly increased from around 65 to over 95 (L*=100 for pure white) and the whiteness of rPET produced from purified BHET (L*=86.21) was greatly improved compared with that from raw BHET (L*=44.01), showing the possibility of achieving the closed-loop recycling of dark PET fibers.